An ammonium salt of a fluorinated bis-sulfonylimide such as a compound represented by the formula (1-1) is known as a salt useful as an electrolyte material excellent in electric conductivity and chemical stability:

As a general method for purifying an ammonium salt, recrystallization is most widely employed. The present inventors have attempted recrystallization of an ammonium salt represented by the formula (1), however, it is specifically difficult to recrystallize an ammonium salt represented by the above formula (1-1), and they have found that purification by a recrystallization method is difficult:[HNR3]+[Q1-SO2—N—SO2-Q2]−  (1)wherein R's which may be the same or different, is a hydrogen atom or a C1-10 alkyl group, and each of Q1 and Q2 which are independent of each other, is a monovalent fluoroorganic group, or Q1 and Q2 together form a bivalent fluoroorganic group.
For example, they have attempted a means of suspending the ammonium salt represented by the formula (1) in a poor solvent, and gradually adding a good solvent with heating to dissolve the ammonium salt represented by the formula (1) in the solvent, followed by cooling for recrystallization. However, the ammonium salt solution after addition of the good solvent did not become a uniform solution but was separated into two phases of a good solvent solution phase of the ammonium salt and a poor solvent solution phase, and recrystallization could not be carried out.
Further, the present inventors have attempted, as another recrystallization method, a method of adding a good solvent to the ammonium salt represented by the formula (1), followed by cooling to precipitate crystals. However, although a crystalline substance was formed, the purity of the crystalline substance was low, and the yield was very low, and thus this recrystallization method could not be employed.
Patent Document 1 discloses a purification method of washing [NH(C2H5OCH3)(C2H5)(CH3)]+[CF3—SO2—N—SO2—CF3]− with an acidic aqueous solution or an alkaline aqueous solution. It is Li, Na, I, Br and the like that can be removed by this method, and even if this method was employed for purification of the ammonium salt represented by the formula (1), the purity was not increased. Further, the decomposition reaction of ammonium represented by the formula (1) occurred.
A lithium salt represented by the formula (3) is a compound which can be used as e.g. an electrolyte for a lithium ion secondary battery. In a case where the lithium salt represented by the formula (3) is used as an electrolyte for a lithium ion secondary battery, high purity of 99 mass % or higher is essential:Li+[Q1-SO2—N—SO2-Q2]−  (3)wherein Q1 and Q2 are as defined for the formula (1).
As a method for purifying a lithium salt, Patent Document 2 discloses a method of reacting [NH(C2H5)3]+[CF3—SO2—N—SO2—CF3]− with an alkali metal hydroxide such as sodium hydroxide to convert it to an alkali metal salt, followed by crystallization.
However, it is a sodium salt that is formed in a case where sodium hydroxide is used, and salt exchange is required to obtain a lithium salt. A method of using lithium hydroxide instead of sodium hydroxide may be mentioned so as to directly obtain a lithium salt, but a solvent is coordinated to the precipitated lithium salt, and a complicated step to remove the solvent is required. Further, if impurities to be removed are reacted with an alkali metal hydroxide, an alkali metal salt of impurities will be formed, and its removal is more difficult.
Patent Document 3 discloses as a method for purifying Li+[CF3—SO2—N—SO2—CF3]−, a method of recrystallizing it from 1,4-dioxane, dissolving it in a polar solvent having a boiling point of at most 100° C., followed by filtration, and removing the solvent. The present inventors have practically conducted this method, and they have found problems in this method such that the yield was so low as 60%, removal of the solvent was very difficult, and removal of the solvent took a long period of time.    Patent Document 1: JP-A-2005-298375    Patent Document 2: JP-A-2000-302748    Patent Document 3: JP-A-9-255685